Mutations in Ras proteins such as KRAS, HRAS, and NRAS are common oncogenic mutations present in human malignancies, including but not limited to colorectal cancer, lung cancer, thyroid cancer, and ovarian cancer. While Ras, in particular KRAS, has been well known as a primary cancer causing protein for more than 30 years, no effective treatments for Ras mutant tumors are currently available. The pharmaceutical industry has invested tremendous resources into the development of Ras and Ras pathway inhibitors with limited success to date. Recently, drugs have been approved targeting kinases involved in signal transduction downstream of Ras (e.g., RAF and MEK kinases). However, even in these cases, their effectiveness in Ras mutant tumors remains to be demonstrated.
Direct targeting of Ras has been deemed unfeasible for many years due to multiple failed attempts and a perception that the protein lacks druggable binding pockets. While crystal structures of Ras are generally consistent with a lack of clearly deep binding pockets, some portions of the protein are highly flexible (switch regions) and may adopt conformations favorable for small molecule drug binding. Exhaustive exploration of chemical space to identify potential direct Ras inhibitors has to date been prevented in part by a lack of robust high throughput assays suitable for screening. Assays used to identify direct Ras inhibitors have either relied on nuclear magnetic resonance (NMR) spectroscopy, or a combination of computational screening or design coupled with relatively low throughput Ras functional assays such as nucleotide exchange or effector binding. These methods do not allow for an unbiased screen of a large compound library. In addition, particularly for NMR and other unbiased binding assays, significant effort is typically made for each hit molecule or class to determine whether the binding event will lead to inhibition of Ras activity.